What you are articulating here can be crystallized into two possibly opposing camps. Even within the ranks of the classical movement you can find these two 'leanings':
Classical Writing obviously belongs to Camp # 1.
You as a mom would have to decide for your particular student(s) whether you think archaicisms would pose a danger or would be a source of enrichment.
The arguments on both sides are many and varied. Camp #1 tends to be adventurous and expansive, Camp #2 takes a defensive stance.
The reality of the situation is that some kids will be fine with either approach. Many kids (in my opinion) will do better with #1 than with #2 because they are presented with a wider variety of sentence structures, vocabulary terms, and generally a richer and more complex English than modern speech and writing provides.
But there is a minority of kids for whom #2 may be a better fit. Kids who struggle with language may not be able to thrive with the older models because they are difficult, and because of their poor judgment in the area of language they may inadvertently adapt poor as well as archaic phraseology if exposed to Camp #1 literature.
The call is yours. However, I would not worry about a kid who is young. Young students seem to memorize whole sentence structures by osmosis. Let them. A bright mind that can recite a whole section from Howard Pyle's
Robin Hood at age 10 is not going to write archaically when he or she is in 11th grade.
On the contrary, most kids by the time they are in high school have developed their own internal sense for how to express themselves on a given occasion. Those who have been exposed to the richer, more varied literature are all the more likely to be able to express themselves better and more suitably for the occasion.
I meet with few students at that age--even some who have been raised on a non-stop diet of CW models and a great deal too much Shakespeare--who cannot hold their own in writing appropriately when it comes to modern styles and modern essays.
But it sounds more to me like you want to make sure you make the right judgment call when you catch an awkward construction in her essays. (is it wrong? is it awkward? or is it archaic?)
Whether to nip it in the bud will depend on the purpose and tone of the essay. If you have chosen an audience for the essay, she has to write to that audience. (We do that in our older books). If there is no specified audience, ask yourself whether your daughter's archaicism is a misplaced anomaly in the middle of an otherwise modern sounding essay, or whether it is a tone that permeates through her whole piece. If she decides to write a whole narrative in the tone of Shakespeare, or in some other stilted voice, this is fine, but the tone more than anything should be consistent and fit the occasion.
My rule of thumb would be this: If you are whipping through one of her essays and one of her constructions 'hits you wrong'.... chances are that you're right. If a construction causes you to pause, ponder, reread and think before moving on, like as not, you can go with your hunch and have her rephrase it, even if you can't put your finger precisely on what it is that makes it sound awkward.
Lene
